LAS SPECIFICATION VERSION 1.4 – R11 - asprs

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A LAS 1.4 file writer who wishes to maintain backward compatibility must maintain both the legacy fields and the equivalent non-legacy fields in synchronization. Of course, this is not possible if the number of points exceeds UINT32_MAX, in which case the legacy fields must be set to zero. If a file writer is not maintaining backward compatibility, the legacy fields must always be set to zero. If there is a discrepancy between a non-zero legacy field and the equivalent LAS 1.4 field, the LAS 1.4 reader should use the legacy value to maintain the same behavior as a LAS 1.1 through LAS 1.3 reader. Best practice is to also throw an informative error so that the file can be repaired. COORDINATE REFERENCE SYSTEM (CRS) REPRESENTATION: GeoTIFF is being replaced by Well Known Text (WKT) as the required Coordinate Reference System (CRS) representation for the new point types (6-10) introduced by LAS 1.4. GeoTIFF is maintained for legacy reasons for point types 0-5. A “WKT” bit has been added to the Global Encoding flag in the Public Header Block. If this bit is set, the CRS for the file will be located in the WKT (Extended) Variable Length Records (EVLR, VLR). A file writer who desires to maintain backward compatibility with legacy LAS for point types 0-5 must add a GeoTIFF VLR to represent the CRS for the file and ensure that the WKT bit is false. The CRS representation is summarized in Table 2 Table 2: Coordinate Reference System Representation Point Type WKT bit == False WKT bit == True 0-5 GeoTIFF WKT 6-10 Error WKT It is considered a file error to have more than one GeoTIFF (E)VLR or more than one WKT (E)VLR in the file. A writer can append a new CRS EVLR to a file by “superseding” the existing CRS (E)VLR. Superseding is performed by changing the LAS_Spec ID of the record to “Superseded”, a new LASF_Spec defined in this release. DATA TYPES: The following data types are used in the LAS format definition. Note that these data types are conformant to the 1999 ANSI C Language Specification (ANSI/ISO/IEC 9899:1999 ("C99"). ● char (1 byte) ● unsigned char (1 byte) ● short (2 bytes) ● unsigned short (2 bytes) ● long (4 bytes) ● unsigned long (4 bytes) ● long long (8 bytes) ● unsigned long long (8 bytes) ● float (4 byte IEEE floating point format) ● double (8 byte IEEE floating point format) ● string (a variable series of 1 byte characters, ASCII encoded, null terminated) American Society for Photogrammetry & Remote Sensing Page 4 of 27 LAS SPECIFICATION Version 1.4-R11
PUBLIC HEADER BLOCK: Table 3: Public Header Block Item Format Size Required File Signature (“LASF”) char[4] 4 bytes * File Source ID unsigned short 2 bytes * Global Encoding unsigned short 2 bytes * Project ID - GUID data 1 unsigned long 4 bytes Project ID - GUID data 2 unsigned short 2 byte Project ID - GUID data 3 unsigned short 2 byte Project ID - GUID data 4 unsigned char[8] 8 bytes Version Major unsigned char 1 byte * Version Minor unsigned char 1 byte * System Identifier char[32] 32 bytes * Generating Software char[32] 32 bytes * File Creation Day of Year unsigned short 2 bytes * File Creation Year unsigned short 2 bytes * Header Size unsigned short 2 bytes * Offset to point data unsigned long 4 bytes * Number of Variable Length Records unsigned long 4 bytes * Point Data Record Format unsigned char 1 byte * Point Data Record Length unsigned short 2 bytes * Legacy Number of point records unsigned long 4 bytes * Legacy Number of points by return unsigned long [5] 20 bytes * X scale factor double 8 bytes * Y scale factor double 8 bytes * Z scale factor double 8 bytes * X offset double 8 bytes * Y offset double 8 bytes * Z offset double 8 bytes * Max X double 8 bytes * Min X double 8 bytes * Max Y double 8 bytes * Min Y double 8 bytes * Max Z double 8 bytes * Min Z double 8 bytes * Start of Waveform Data Packet Record Unsigned long long 8 bytes * Start of first Extended Variable Length Record unsigned long long 8 bytes * Number of Extended Variable Length unsigned long 4 bytes * Records Number of point records unsigned long long 8 bytes * Number of points by return unsigned long long [15] 120 bytes * Any field in the Public Header Block that is not required and is not used must be zero filled. File Signature: The file signature must contain the four characters “LASF”, and it is required by the LAS specification. These four characters can be checked by user software as a quick look initial determination of file type. American Society for Photogrammetry & Remote Sensing Page 5 of 27 LAS SPECIFICATION Version 1.4-R11
Page 1 and 2: LAS SPECIFICATION VERSION 1.4 - R11
Page 3: 2 Conformance The data types used i
Page 7 and 8: System Identifier: The version 1.0
Page 9 and 10: Table 6: Variable Length Record Hea
Page 11 and 12: Table 8: Classification Bit Field E
Page 13 and 14: Item Format Size Required Number of
Page 15 and 16: Z = Z0 + Z(t) where X, Y and Z are
Page 17 and 18: Scanner Channel: Scanner Channel is
Page 19 and 20: Item Format Size Required User Data
Page 21 and 22: Item Format Size Required Record Le
Page 23 and 24: struct sGeoKeys { unsigned short wK
Page 25 and 26: The bit mask in the options field s
Page 27: These records contain information t

LAS SPECIFICATION VERSION 1.4 – R11 - asprs

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